1. Field of the Invention
The present invention relates to a formation method of a water repellent layer and to an injector having the water repellent layer.
2. Description of Related Art
There is known an injector that injects fuel directly into a cylinder of an internal combustion engine, for example, as described in Patent document 1 (JP-A-H10-159688). This type of injector has an injection hole forming section that forms an injection hole. The injector injects the fuel from the injection hole into a combustion chamber of the cylinder directly under the injection hole.
According to the technology of the injector described in Patent document 1, an inner periphery of the injection hole and a circumference of the injection hole in the injection hole forming section are coated with a FAS coating consisting of fluoroalkyl silane (hereafter, referred to as FAS). According to the technology, the FAS coating has water repellency because of existence of fluoroalkyl groups and enables inhibition of adhesion of deposits to the inner periphery of the injection hole and the circumference of the injection hole.
However, there is a concern that the adhesion inhibition function to inhibit the deposits, which are generated in the combustion chamber, from adhering to the injection hole falls for a following reason in the conventional technology of Patent document 1. That is, the FAS coating having the water repellency is coated on the injection hole, and the injection forming section is in an environment where the injection hole forming section is exposed to a hot gas in the combustion chamber. Therefore, a surface of the injection hole forming section on the combustion chamber side is brought to a high temperature state, and therefore the FAS coating degrades thermally. If the thermal degradation of the FAS coating occurs, the deposit adhesion inhibition function to inhibit the deposits from adhering to the injection hole falls.
Studies performed by the inventors of the present invention about the above-described FAS coating revealed that the fluoroalkyl group consisting of an organic macromolecule containing the fluorine starts thermal decomposition at relatively low temperature (approximately 250 degrees C.) as compared to temperature on the combustion chamber side surface of the injection hole forming section. The inventors think that the fall of durability concerning heat resistance is caused because the water repellent function of the FAS coating is provided by the above-described organic macromolecule.